1.The enthalpy of mixing of water and tetraethylene glycol monooctyl ether (C8E4) was precisely measured as a function of concentration of surfactant and temperature by using the high accuracy isothermal titration calorimeter. The differential enthalpies of solution of monomer and micelle and the enthalpy of micelle formation were evaluated by applying the thermodynamic relations developed to the enthalpy of mixing measured. The dissolution of pure liquid C8E4 is exothernmic and the micelle formation is endothermic, respectively. It is shown that the partial molar enthalpy of water may afford an insight into the hydration in micellar state. It was also shown that there exists the difference in values between the enthalpy of micelle formation obtained directly from calorimetry and that indirectly from the temperature dependence of critical micelle concentration.2.The experimental method is now applied to the measurement of the enthalpy of mixing of mixed surfactant systems. Furthermore it was suggested that this calorimeter, because of its extremely high resolution, is applicable to measure directly the enthalpy of interface formation and adsorption of surfactant at the interface.